The present technology relates generally to the field of radiopharmaceuticals and their use in nuclear medicine as tracers, imaging agents and for the treatment of various disease states. It is well known that tumors may express unique proteins associated with their malignant phenotype or may over-express normal constituent proteins in greater number than normal cells. The expression of distinct proteins on the surface of tumor cells offers the opportunity to diagnose and characterize disease by probing the phenotypic identity and biochemical composition and activity of the tumor. Radioactive molecules that selectively bind to specific tumor cell surface proteins provide an attractive route for imaging and treating tumors under non-invasive conditions. In particular, the present inventors have found that radiolabeled ligands to the CA-IX protein, often over-expressed on many cancer cells provide an attractive route for non-invasive imaging and selective targeting of cancer cells.
Carbonic anhydrase (CA) or carbonate dehydrases are a family of enzymes that catalyzes the rapid conversion of carbon dioxide to bicarbonate and proton in the presence of water. Carbonic anhydrase, therefore, play an important role in maintaining the acid-base balance (pH), in blood and tissues and also play a role in transporting carbon dioxide out of tissues. CA is a zinc metalloenzyme, the active site zinc being coordinated to the imidazole residues of three histidine side chains.
There are at least 16 isozymes in the carbonic anhydrase family. Specific isozymes are found either in the cytosol, anchored to the membrane, within the mitochondria, or secreted from the cell. The well studied constitutively expressed isozyme, carbonic anhydrase II (CA-II), is found in the cytosol of most cell types, and is the primary isoform responsible for the regulation of intracellular pH.
CA-IX is a membrane-anchored isoform of the enzyme with its catalytic domain in the extracellular space. It has a limited tissue distribution and is found at low levels primarily in the gastrointestinal tract. The expression of CA-IX is under the control of HIF-1α, and this isozyme is highly expressed in tumors cells exposed to hypoxia both in vitro and in vivo. Increased CA-IX expression has been detected in carcinomas of the cervix, ovary, kidney, esophagus, lung, breast, and brain. The low extracellular pH as a result of the activity of CA-IX leads to tumorigenic transformation, chromosomal rearrangements, extracellular matrix breakdown, migration and invasion, induction of growth factors, protease activation, and chemoresistance. Accordingly, a correlation can be made between the cellular levels of CA-IX and tumor progression. Radiopharmaceuticals directed to the CA-IX protein thus provide an novel avenue for the non-invasive treatment of cancer.
The selective targeting of cancer cells with radiopharmaceuticals, either for imaging or therapeutic purposes is challenging. A variety of radionuclides are known to be useful for radioimaging, including Ga-67, Tc-99m, In-111, I-123, and I-131. The preferred radioisotope for medical imaging is Tc-99m, because it has a short (6 hour) half life, is readily available at relatively low cost and emits gamma-photons of 140 keV. Moreover, Tc-99m complexes, such as, water and air stable Tc(I) complex [99mTc(OH2)3(CO)3]+ complex can be readily prepared in saline under 1 atm of carbon monoxide (CO). Accordingly, bifunctional molecules that comprise a specific receptor honing bioactive molecule covalently tethered to a 99mTc or 186/188Re coordination complex provide a novel system for the selective imaging and targeting of cancer cells.